A dynamic-quantity semiconductor sensor (i.e., a physical quantity sensor) of prior art comprises a base plate and a semiconductor layer. The semiconductor layer is supported on the base plate. Through trenches are made in the semiconductor layer by etching. The semiconductor layer has a movable part, which is defined by the trenches and released from the base plate. The sensor is, for example, disclosed in Japanese Patent Application Publication No. 2001-91265.
The dynamic-quantity semiconductor sensor detects the applied dynamic quantity, i.e., the physical quantity, based on the displacement of the movable part when dynamic quantity such as angular velocity or acceleration is applied to the sensor.
For example, Japanese Patent Application Publication No. 2001-133268 discloses an angular-velocity sensor which is made of an SOI (Silicon-On-Insulator) substrate consisting of two silicon plates and an oxide film, the oxide film sandwiched between the silicon plates.
The above sensors of prior art are so-called dynamic-quantity semiconductor sensors of the surface-processing type. They are made as follows. One of the two silicon plates of an SOI substrate is a supporting base plate and the other is a semiconductor layer. Trench etching is made from the top-surface side of the semiconductor layer to form the pattern of a structure including a movable part. Then, the movable part is released by removing the lower part of the semiconductor layer by side etching or removing the oxide film by sacrifice-layer etching.
Besides, a plurality of through holes is made in the movable part to raise the efficiency of etching and reduce the weight of the movable part. This is disclosed in, for example, Japanese Patent Application Publication No. 2001-99861.
The above dynamic-quantity semiconductor sensors of prior art have the following problems. In the step of releasing the movable part by etching from the through holes, the finished shapes of various parts of the movable part become considerably uneven, the characteristics of movable parts vary, and the strength of the movable part deteriorates.